With the growth of the personal computer (PC), the PC color monitor has become very popular. The PC monitor differs from a TV set in that it has higher resolution, does not have a tuner, and has separate TTL video and sync signals. The separate RGB signals ae essential to produce the high resolution necessary to display small, readable text on the monitor. The composite signal used for color TV is limited to a 3.58 MHZ video bandwidth due to the nature of the NTSC signal. A standard PC uses a 14.318 MHZ dot clock for its video, thereby making a composite signal impractical.
To cut down on cost and the lack of need for many colors, a standard PC color monitor can only display 8, 16 or 64 separate colors, depending on the PC and monitor. The video signas are digital (either on or off) as opposed to the analog (infinite color availability) signal generated for TV.
It was believed that an RGB TTL monitor cannot be used to generate the infinite shades of color necessary for TV use. The Analog To Digital Video Adapter does just that. The invention converts the NTSC analog signal to pulse width modulated TTL RGB video signals, and produces infinite colors and TV quality to an 8 color monitor.
The invention requires that an NTSC standard color signal is supplied to it since it does not contain a tuner or receiver. All video cassett recorders (VCR) have a tuner and virtually all of them have a composite video output that will interface to the invention. The popularity of the VCR, along with the PC, makes the invention the perfect device to inexpensively add a TV to the home or office that already have a PC monitor and a VCR.
The prior art demonstrates how to make an 8 color (RGB digital) color monitor to perform as a 64 color (R1,R2,G1,G2,B1,B2 digital) monitor when the monitor is driven from a computer; see Wahlquist in U.S. Pat. No. 4,516,118. The prior art converts the R1,R2,G1,G2,B1,B2 digital video signals into RGB signals by pulsewidth modulating the RGB signals using the computers video clock. The method used in the prior art cannot be modified to convert analog signals to have an 8 color monitor produce the infinite colors necessary to display a TV image. Its limiting factor is the use of the video dot clock for the pulsewidth modulation. In order to obtain infinite colors an infinite dot clock would be required, which is impossible. At best, the prior art method would produce a grainy looking display i.e., where a smooth transition of a particular color is required, a staircase transition of brightness would occur due to the limiting number of colors (brightness levels) it can produce. Even if a theoretical video clock was generated at an infinite frequency, and theoretical integrated circuits were invented, the prior art method would not be practical due to the number of components required and the cost involved. The present invention overcomes the shortcomings of the prior art by comparing a reference analog ramp voltage with the analog video signal which produces a digital signal whose pulsewidth is proportional to the video signal's amplitude, thus allowing an infinite number of pulsewidths which corresponds to an equal number of colors. The invention is an extremely efficient method to increase the number of colors that can be displayed on an 8 color monitor.